1. Field of the Invention
The present invention relates to a variable capacitor and more particularly to improvements therein for an increased stability of temperature characteristics of capacitance and for an increased capacitive efficiency, providing a structure which can be advantageously utilized in, e.g., a small trimmer capacitor.
2. Description of the Prior Art
The prior art of interest to the present invention is shown in FIG. 1. Referring to FIG. 1, the prior art variable capacitor comprises a cylindrical insulating cap 1 which houses a disc-like nonmetallic elastic member 2, a disc-like rotor electrode leading-out member 3 having a depending terminal portion 3a, a rotor electrode 4, a disc-like dielectric plate 5, and a disc-like stator 7 on which a semicircular stator electrode 6 having depending terminal portion 6a integral therewith is integrally formed to be flush therewith. A bottom opening 1a of cap 1 is integrally secured to an insulating base 8 having throughgoing apertures 8a for said rotor terminal portion 3a and stator terminal portion 6a.
Variable capacitors, such as the one described above, require a movable rotor electrode and a fixed, stator electrode, with a dielectric intervening therebetween. Accordingly, as shown in FIG. 1, the rotor electrode 4 and stator electrode 6 are disposed in opposed relation to each other with the dielectric plate 5 interposed therebetween, the rotor electrode 4 being rotatable, the stator electrode 6 being fixed. Such a conventional variable capacitor is disclosed in, e.g., U.S. Pat. No. 4,101,951 issued to Saburo Kuze on July 18, 1978 and assigned to Murata Manufacturing Co., Ltd., the assignee in the present application. This variable capacitor, however, exhibits the following drawbacks.
Since the electrodes are respectively disposed in contact with the main surfaces of the dielectric plate, there will be a slight clearance and therefore a layer of air between each electrode and the associated main surface of the dielectric plate. As a result, there is a decrease in capacitance. Further, since the layer of air varies in size with temperature variations, the temperature characteristics of the capacitor are unstable. Accordingly, considerably strict polishing has heretofore been necessary on the mutual contact surfaces of the dielectric plate and electrodes.
Further, with the arrangement shown in FIG. 1, it has been necessary to apply resin coatings to the throughgoing apertures 8a after the terminal portions 3a and 6a are passed through the throughgoing apertures 8a of the insulating base 8 since there is a danger of the flux flowing into the insulating cap 1 through the throughgoing apertures 8a when soldering the terminal portions 3a and 6a. The number of the component parts is large and the number of steps of assembly operation is increased, thus adding to the cost. The presence of a large number of places where the component parts contact each other has made it difficult to achieve accuracy in assembly and adjustment.
Since the dielectric plate 5 is not positively fixed but rotatable, it is possible that when the rotor electrode 4 is rotated for adjustment through an adjusting aperture 1b formed in the top lid of the insulating cap 1, the dielectric plate 5 will be broken off or cracked as it is concomitantly rotated.
While the variable capacitor has various problems, it has its merits which cannot be overlooked. More particularly, the rotor electrode 4 can be rotatably supported simply by being received in the cylindrical insulating cap 1, requiring no rotatable shaft member to be additionally provided. This is an important merit particularly in the case of a tiny trimmer capacitor. An arrangement including a rotatable shaft member for rotatably supporting the rotor electrode thereon would be relatively complicated, requiring additional component parts and additional steps of assembly operation therefor. It is therefore desired to solve the above-described problems while making use of the foregoing merits of the prior art. To this end, the variable capacitor of the present invention basically includes a rotor electrode supporting mechanism as shown in FIG. 1.
The problems described above are primarily caused by the fact that the dielectric plate is not positively fixed but concomitantly rotatable. If the dielectric plate is positively fixed, most of the foregoing problems are solved. Insofar as an arrangement wherein the dielectric plate is fixedly provided is concerned, a technique of interest is disclosed in U.S. Pat. No. 2,475,144 "Capacitor" issued to George T. Kodama et al on July 5, 1949. The capacitor disclosed therein has a plurality of combinations of capacitance, such as a fixed capacitance and a variable capacitance. Thus, the capacitor structure comprises a body of insulating material bearing connector or terminal elements for contact with stationary conductive plates on a dielectric plate which may be readily assembled with or disassembled from the body. For use as a fixed capacitor, a second stationary conductive plate is applied on another side of the dielectric, and, for use as a variable capacitor, a rotor plate member is held in contact with an unplated portion of the dielectric. Assembly of the parts is made simply by the interrelated shaping of the parts and the provision of a stop limiting relative movement of the parts in one direction. A resilient member presses and locks all the assembled elements in a given position in the body.
Stated in more detail by reference to the drawings attached to the above described Kodama et al patent, a body member 10 is prepared which is preferably formed with flanges 11 extending laterally a relatively large and material distance from the body and severally having a portion 12 extending at right angles to the flanges 11 and with flange edge portions 13 extending from the flange portions 12 toward each other, the several flange portions defining an open-ended groove with reentrant or undercut side walls. A rectangular dielectric member 30 is inserted into the open-ended groove of the body member 10 from one end thereof, where it is snuggly fitted and physically fixed. In this manner, the dielectric member 30 is fixedly installed.
The member 30 is first provided with plates 31 and 32 for a fixed capacitance and additionally with a plate 33 for forming a stator plate for a variable capacitance. It is to be noted that the plate 30 is of a casual nature in that it is provided in passing when the plates 31 and 32 for a fixed capacitance are formed. Thus, the major role of the dielectric member 30 is in any sense to form a fixed capacitance. The dielectric member 30 is protected by the body member 10. This manner of protection implies the fixedness of the dielectric member 30. The fixed state is further ensured by a presser member 41. A rotatable member 36 provided with a plate portion 37 which cooperates with the stator plate 33 formed on the dielectric member 30 to form a variable capacitor is rotatably mounted on the presser member 41 through a boss 38. Therefore, the rotatable member 36 inherently includes the boss 38 serving as a rotatable shaft.
Such arrangement will impair the merits of the variable capacitor having a cylindrical case shown in FIG. 1 described above. More particularly, it complicates the structure, increases the number of component parts and the number of steps of assembly operation and involves intricate manipulation in assembly operation. Therefore, it is not at all suitable for use as a variable capacitor structure such as a tiny trimmer capacitor. Further, the arrangement wherein the dielectric member 30 is inserted into the body member 10 in a direction in which its surface extends is not at all suitable for a variable capacitor with a cylindrical case such as the one shown in FIG. 1.